The invention relates to a procedure for diagnosing the velocity and the dead time of an exhaust gas probe, which is arranged in an exhaust gas duct of a combustion engine, whereby the diagnosis is carried out on the basis of a comparison of a modeled and a measured signal after a default change of a fuel air ratio of an air fuel mixture that is supplied to a combustion engine and whereby the signal is an output signal of the exhaust gas probe or a modeled or measured signal that is derived from the output signal.